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03-11-2013, 01:55 AM   #1
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Current lenses on a FF

I've been carefully reading all the discussions and so forth on the (hopefully) up coming Pentax full frame.
However I'm still confused as what this will mean for my current collection of lenses.
Now, I'll assume for a sec that Pentax wont do anything stupid like chance the K-Mount, so physically (I hope) lenses will still fit. But what will it mean?
Will my lenses just change they're designated focal length? (eg a 50mm will actually work like a 35mm) Will there be horrid vignetting? Or is it like dividing by zero and the world will implode?

Cheers,
Lurch

03-11-2013, 02:32 AM   #2
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Focal length does not change. Field of view does. So to fit the same scene into the frame you will need a lens 1.5x the focal length. (For those of us who grew up with 35mm film, lenses go back to their 'proper' field of view.)

DA lenses are only designed to cover the APS-C sensor so yes there will be very severe vignetting issues with some of them, and failing that edge softness on some of the rest. I think there's a few threads here on testing DA lenses for FF performance (using film bodies). Some of them actually seem to be OK - by liuck or by virtue of recycled FF optic designs perhaps?

FA, A, M, K, M42 lenses should all be fine as a FF sensor (by definition) covers the same area as the 35mm film frame.
03-11-2013, 02:38 AM   #3
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Ok; thanks for that. Any thought on Sigma stuff? The rest I dont really care about except my much loved 20-40mm
03-11-2013, 02:39 AM   #4
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QuoteOriginally posted by Lurch Quote
I've been carefully reading all the discussions and so forth on the (hopefully) up coming Pentax full frame.
However I'm still confused as what this will mean for my current collection of lenses.
Now, I'll assume for a sec that Pentax wont do anything stupid like chance the K-Mount, so physically (I hope) lenses will still fit. But what will it mean?
Will my lenses just change they're designated focal length? (eg a 50mm will actually work like a 35mm) Will there be horrid vignetting? Or is it like dividing by zero and the world will implode?

Cheers,
Lurch
Sensor size doesn't change focal length, just field of view. On FF, your FF compatible lenses will have the field of view native to their focal length (while on crop it is different). For example, on FF 50mm FOV is 40 degrees, on crop - 20 degrees.

Check this thread about vignetting. https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/31629-da-lens-...ts-thread.html

03-11-2013, 02:55 AM   #5
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Ahh, ok. So its the increased FOV therefore that causes vignetting/soft edges?
03-11-2013, 03:02 AM   #6
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I believe that vignetting appears when sensor size is too big for lens "hole" to fully cover.
03-11-2013, 03:07 AM   #7
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Ok; it sounds for a lot of lenses, it will be a case of 'suck it and see'
03-11-2013, 05:38 AM   #8
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QuoteOriginally posted by Lurch Quote
Ahh, ok. So its the increased FOV therefore that causes vignetting/soft edges?
Picture this- or even try it. Hold a piece of paper a few centimetres behind a camera lens and it will project an image of the scene on the paper. Hold it at just the right distance and it will be in focus. (If you want to actually try this you will need a bright scene and an otherwise darkly lit piece of paper, e.g. in a dark room with the lens pointing through a small opening through a blind, out a window.

The image will have a brightest point and will fall off from there. A FF lens should make an image that covers a 35x24mm frame with minimal fall-off. A lens designed for digital - including Sigma DC series - may potentially only cover a smaller APS-C sensor area. Or even if it extends further, it may have poor IQ out there.

03-11-2013, 06:30 AM   #9
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QuoteOriginally posted by Lurch Quote
Current lenses on a FF
I'm just lucky all my current lenses all FF.
01-06-2014, 05:56 AM   #10
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TS, you may like to check out this thread I started

https://www.pentaxforums.com/forums/169-pentax-full-frame/246917-does-my-m42-...ploration.html




Hold on to those FF lenses
01-06-2014, 10:17 PM - 1 Like   #11
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You can easily test the vignetting yourself. Get a cheap (even broken) film Pentax SLR and mount whatever K-mount lens you want. The DA 18-55mm kit lens easily shows vignetting at 18mm through the viewfinder.

You'll probably pay more for S&H than the actual camera itself. Let me know if you want one for grins.
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